Phosphorus exists in waters as dissolved ortho-phosphate, polyphosphate, and complex organo-phosphorus compounds. In typical phosphorus-containing waste waters, such as the secondary or tertiary effluents of municipal waste water treatment plants, there is a dissolved fraction of phosphorus compounds, primarily in the form of ortho-phosphate and poly-phosphates, and a suspended fraction of micro-particulate phosphorus-containing solids. Trace levels of arsenic are sometimes found in some sources of drinking water and in higher concentrations in some waste waters. Arsenic can occur in natural waters as reduced arsenite, As(III), or oxidized arsenate, As(V).
Several methods are currently utilized for the removal of phosphorus compounds, arsenic, and other contaminants from waste water. Micro-particulate and other solid contaminants are typically removed by filtration using a solid media such as sand, and sedimentation, where solid contaminants with higher densities than water are allowed to settle. Dissolved contaminants are typically removed by flocculation and sorption. In flocculation, metal salt solutions are mixed with waste water to precipitate the contaminant out of solution, where it can then be removed through filtration or sedimentation. In sorption, contaminated waste water is passed through a stationary filtration media, typically iron oxide coated sand, having a partially charged cationic boundary layer that is reactive with a target contaminant dissolved in the waste water.
In conventional fixed-bed filtration systems, filtration media can quickly lose its filtration efficiency as the interstitial spaces between the particles of the filtration media become saturated with micro-particulate and solid contaminants. Thus, the filtration media must be flushed or replaced, which tends to be costly and time intensive. Additionally, the filtration process must be stopped while the filtration media is being flushed or replaced.
Moving-bed filtration devices seek to mitigate these limitations by utilizing processes that remove micro-particulate and solid contaminants from the filtration media while simultaneously filtering water. These moving-bed filtration devices still have a disadvantage in that they do not remove dissolved contaminants from waste water.
U.S. Pat. No. 5,369,072 to Benjamin et al. describes methods of preparing iron-oxide coated sand to be used as a filtration media in either fixed-bed or moving-bed systems to remove both solid and dissolved contaminants from waste water. When such a filtration media is used in a fixed-bed system, there remains the disadvantage of having to flush or replace the filter media on a regular basis to remove the solid contaminant waste. Although the use of this media in a moving-bed system may overcome these disadvantages, it does not overcome a disadvantage common to the use of the described iron oxide coated sand in either system. That is, the reactive surface of the iron oxide coated sand becomes saturated with adsorbed contaminants, and therefore needs to be either replaced or regenerated. Benjamin states that the adsorbed contaminant can be desorbed by treating the saturated filtration media with a solution with a pH range that is known to desorb the specific contaminant ion from the specific adsorbing surface. Therefore, in either a fixed-bed or moving-bed system, filtration of water must stop to allow the filtration media to be rinsed with pH solution.
Environmental concerns and increasingly stricter government regulations have many industries searching for cost-effective and efficient water-treatment solutions. Additionally, there is a recognized need to be able to filter out water contaminants that exist in what may be considered trace amounts. For example, the removal capability of conventional coagulation-precipitation methods drop off significantly at contaminant levels lower than 500 parts per billion (ppb). However, science has recognized that ambient phosphorus levels in water greater than 10-20 ppb can lead to eutrophication. The U.S. Environmental Protection Agency is pushing for lower limits of phosphorus in effluents. Currently, the EPA estuarine water criteria for total phosphorus is 0.10 mg/L. High volume dischargers are experiencing a reduction in the levels allowed in their regulatory permits.